Phase locked loops are commonly used on various circuit applications, and may be used as clock multipliers. For instance, an input clock of 10 MHz can be multiplied by a phase locked loop to yield a signal at 1 GHz, preferably in phase alignment with the 10 MHz clock signal.
A typical phase locked loop circuit 10 is shown in FIG. 1, wherein a 20 MHz clock 12 is divided by a value M (i.e., M=2 to provide a 10 MHz reference signal in this example) and is fed into the components 14 of a typical phase locked loop. These components include a phase/frequency detector 16, a filter 18 (typically, made of a charge pump and a filter), a voltage controlled oscillator (VCO) or variable frequency oscillator (VFO) 20, and a divider 22 (shown to divide by N). The phase/frequency detector 16 sends the filter information about the frequency and phase of the reference signal 24 relative to the feedback clock signal 26. The filter 18 integrates this information into a voltage. The VCO 20 converts the voltage information into a higher speed/frequency output signal, which is fed back into the phase/frequency detector 16 through the divider 22. The divider takes the higher speed frequency 28 and divides it down for comparison to the reference signal 24 by the phase/frequency detector 16.
Low noise phase locked loops use LC-types of voltage controlled oscillators (VCO) in order to achieve high performance or high speed operations. Unfortunately, LC-types of voltage controlled oscillators have, in general, a limited frequency range of operation. Hence, LC-types of voltage controlled oscillators may be difficult to use with products or circuits needing wider frequency ranges, or fabrication processes which result in variations that necessitate wider frequency ranges.
In order to provide voltage controlled oscillators with wide frequency ranges, conventionally voltage controlled oscillators with a large amount of gain (MHz/V) can be used. However, for such conventional voltage controlled oscillators, the high gain stages subject the voltage controlled oscillator to a greater sensitivity to noise. In other systems, multiple voltage controlled oscillators can be used in an attempt to provide a wide frequency range of operation, however such designs require large areas in the semiconductors. Other voltage controlled oscillators can be calibrated prior to turning on a phase lock loop associated therewith in order to provide a wide frequency range of operation, however this operation may take time and power and may not be sufficiently robust if temperature or other process parameters change during operations.
As recognized by the present inventors, what is needed is a phase locked loop circuit can operate over a wide range of input frequencies.
It is against this background that various embodiments of the present invention were developed.